Assessment of the properties of the circadian pacemaker and its functioning in organisms is often dependent on analysis of the various output rhythms generated by a central oscillator. Often, sophisticated time series analysis (TSA) is required to identify the rhythmic components (i.e., intrinsic period and endogenous phase) in the data collected. Each estimate obtained through TSA must be tested for validity so that even subtle discriminations are made with statistical confidence that is standard across all projects. Formulation of mathematical models to describe the physiological systems can assist in providing both testable predictions for experimentation and interspecies comparisons. Use of a common language for presentation of data allow for common modeling procedures and also facilitates the exchange of result between projects. Several lines of evidence indicate that across mammalian species aging causes the pacemaker to speed up, possibly by a reduction in the cell count of the SCN. Aging might then be expected to weaken pacemaker influence, but the evidence here is less robust, partly because rhythm strength is more difficult to quantify than rhythm period. It is in devising methods for assessing pacemaker strength that models will be especially valuable. The basic concept will be to measure the pacemaker against other physiological processes with which it interacts. These other process must themselves be model some will be clearly rhythmic while others may be essentially random. Quantification of the various influences which the pacemaker can exert will provide a direct measure of its physiological significance. Services from the Analytic core will be available on-site for each project investigator through a common programming language, and exchange of data between laboratories will be facilitated by data management assistance. The Analytic Core will establish standards for graphic presentations to enable investigators to readily understand results for comparison across projects. Analysis requiring statistical formulation, TSA or mathematical modeling modified for specialized objectives will also be provided by the Analytical Core.